Polyester resins, and among these particularly alkyd resins are one of the most common binder class used for ambient-cure, solvent-based coatings. The resistance properties of traditional solvent-borne alkyd resins are developed via autooxydative crosslinking of the paint film when subjected to air. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidised in air to the corresponding hydroperoxides which subsequently decompose to generate free radicals, thus resulting in an oxidative crosslinking process. This oxidative crosslinking process is usually accelerated by adding the so-called driers, such as, for example, various salts of cobalt, zirconium, calcium, and manganese. However, alkyd resins have relatively slow “dry” or cure times, particularly at ambient temperatures. Various modifications have been made to alkyd resins to address such concerns.
One such attempt includes modification of a polyester resin or an alkyd resin with a vinyl or acrylic polymer, based on monomers such as styrene or methyl methacrylate, via a free radical polymerisation of these monomers in the presence of the alkyd resin, to produce a vinyl-alkyd copolymer or a so-called “vinylated alkyd” or “vinylated polyester”.
Vinylated alkyd resins generally have a higher molar mass and a higher glass transition temperature (Tg), leading to coatings with reduced tack-free time. However, the so-called “through-dry time” (complete oxidation of unsaturation in the fatty acids forming the alkyd part of the modified resin) of such coatings is longer than in non-modified alkyds due to the decreased degree of unsaturation in the alkyd as a result of copolymerisation with the vinyl monomers. An additional drawback is that paint formulations containing vinylated alkyd resins require greater amounts of solvent, due to the increased molar mass and Tg of the vinylated alkyd.
Water-borne alkyd resins are applied in the form of aqueous emulsions or dispersions, and only need small quantities of coalescing additives. Evaporation of water and oxidative drying of the alkyd, however, occur on the same time scale as in solvent-borne alkyds. While other resin systems allow faster curing, the favourable properties of alkyd resins particularly on wood substrates, such as wood wetting (“Anfeuerung”), make the use of alkyds in this application field highly desirable.
PCT/EP2009/063151 provides a modified polyester or alkyd resin made by reaction of a hydroxy functional polyester or alkyd resin with an olefinically unsaturated compound having reactive groups which are reactive towards hydroxyl groups in the said polyester or alkyd resin, and by dispersing this modified polyester or alkyd resin in an aqueous medium using an emulsifier which is compatible with the said modified polyester or alkyd resin, to form a radiation-curable composition which can be radically polymerised following irradiation with high-energy radiation such as ultraviolet light or electron beams. PCT/EP2009/063151 is incorporated herein by reference, in particular with respect to the patent claims.
It has now been found that it is highly advantageous to add to the radiation curable aqueously dispersed polyester resin composition (1) based on polyester resins or alkyd resins having olefinic unsaturation as described in PCT/EP2009/063151 at least one other olefinically unsaturated compound (2) selected from the group of: water-dispersible compounds (21); water-soluble compounds (22); and/or water-compatible compounds (23), wherein said water-compatible compounds (23) have a weight average molecular weight of at least 450 Dalton and are different from the reaction product AC.